Somitogenesis

Question 1

What is the axial musculoskeletal system?

Answer 1

• includes the vertebrae and ribs, the muscles attached to them and dermis overlying them
• during development it also includes the muscles of the limb but not the bones
• all of these tissues come from the embryonic somite

Question 2

what three main components do somites form?

Answer 2

• axial skeleton
• skeletal muscle
• dermis

Question 3

what are somites and where are they located?

Answer 3

• somites are segments of tissue along the body axis
• located on either side of the neural tube

Question 4

What is somitogenesis?

Answer 4

• the formation of somites

Question 5

What is the name of the mesenchymal tissue that forms somites?

Answer 5

• pre-somatic mesoderm (PSM) forms somites

Question 6

what happens at the cranial end of the PSM and the caudal end?

Answer 6

• at the cranial end of the PSM somites from
• at the caudal end of the pre-somatic mesoderm cells continue to divide and lengthen the region

Question 7

How many somites does it take to form one singular vertebra?

Answer 7

• one pair of somites = 1 vertebra

Question 8

How long does it take for one pair of somites to segment from the pre-somatic mesoderm (in chicks)?

Answer 8

• 90 mins providing the temperature conditions are optimal

Question 9

Segmentation of the PSM is dependent on a molecular clock, what is the name of the molecular clock?

Answer 9

• time measured on the molecular clock - a gene called hairy
• e.g., ach cycle if hairy expression lats 90 mins so a somite forms every 90 mins in a chick

Question 10

What happens at the cranial end of the PSM in order for somites to from?

Answer 10

• cells at the cranial end of the pre-somatic mesoderm undergo a mesenchymal to epithelial transition to form somites

Question 11

Where are the most developed somites found?

Answer 11

• most developed somites found at the cranial end of the PSM

Question 12

what 3 tissues can somites from?

Answer 12

• sclerotome
• myotome
• dermatome

Question 13

What part of the body is formed from the sclerotome?

Answer 13

• the axial skeleton (vertebrae and ribs)

Question 14

What must be formed from the somite before the myotome and dermatome can form?

Answer 14

• the dermatomyotome

Question 15

What is formed from the myotome and dermatome?

Answer 15

• myotome = forms the skeletal muscle in the body and limbs
• dermatome = forms the dermis

Question 16

How and where is the epithelial ball (the somite) formed?

Answer 16

• the PSM undergoes a mesenchymal to epithelial transition in caudal immature somites

Question 17

How and where is the sclerotome formed?

Answer 17

• In maturing cranial somites, the ventromedial portion of the somites undergoes and epithelial to mesenchymal transition to form sclerotome

Question 18

In the maturing somite, after sclerotome formation, the dorsal aspect remains epithelial and is called the dermomyotome - what do the dermomyotomal cells the go on to from?

Answer 18

• dermomyotomal cells go on to form the myotome

Question 19

Can myotomal cells divide?

Answer 19

• myotomal cells can NEVER divide
• cells continually recruited from the dermomyotome

Question 20

How is the dermatome formed?

Answer 20

• once all of the myotomal cells have ingressed from the dermomyotome, the cells remaining in the dermomyotome become mesenchymal tissue - the dermatome

Question 21

Describe re-segmentation?

Answer 21

• Sclerotome re-segments. Fissure develops in middle of sclerotome (A-P).
• Posterior half of one half of somite fuses with anterior half of adjacent sclerotome.
• Myotome does not re-segment. Therefore, muscle binds adjacent segments.
• Sclerotome encases neural tube and sclerotome differentiates first into cartilage and then bone. Forms vertebrae.